[TowerTalk] Ladder Line Question?

[jimlux]

On 4/21/19 2:36 PM, Jim Brown wrote:
> Hi Mike,
> 
> I see several possible issues. Like Jim Lux, I don't trust the loss data 
> nor the measurement technique. Far better would be swept S11 
> measurements of a known length with the far end open and shorted, and 
> post-processing in AC6LA's ZPlots. From my own experience attempting to 
> measure paired lines, the most critical part is dealing with the 
> interface between an unbalanced analyzer and the line. KN5L does this 
> with a carefully wound transformer at the output of the analyzer, with 
> calibration performed on the output side of the transformer (that is, 
> where the line is connected).


Cal standards wouldn't be too tough for this - You'd need a short 
(easy), open (easy), and a load - a bit tougher, but getting a 
non-inductive resistor of a few hundred ohms with short wires on it 
isn't that hard.

Then you could at least move the measurement plane to the start of the 
"line under test".

However, warts and all, I think the measurements in the QST article are 
useful - the loss is fairly low and matches theoretical expectations, 
since the fields are "close" to the conductors.




> 
> The other possible issue is absorption of rain by the foam, and it's 
> effect on characteristics of the line. Many years ago, QST published 
> excellent work by N7WS on the greatly increased loss of window line when 
> it is wet. That piece is reprinted in one of the ARRL Antenna Compendiums.
> 
> 73, Jim K9YC
> On 4/21/2019 12:10 PM, Michael Tope wrote:
>> This looks like a promising technique, but I suspect it would have to 
>> been encased in some sort of outer sheath (like PVC pipe) when used 
>> outdoors to protect it from the sun and rain.
>>
>> 73, Mike W4EF
>>
>> On 4/13/2019 9:05 AM, jimlux wrote:
>>> On 4/13/19 8:44 AM, John Pieszcynski wrote:
>>>> See "A Novel Approach to Using Window Line" by W6NBC, QST August 2018.
>>>>
>>>> The article is on his website @
>>>> http://www.w6nbc.com/articles/2018-08QSTladderline.pdf
>>>>
>>>> 73,
>>>> John W2FV
>>>>
>>>
>>> Fascinating data..
>>>
>>> I would have liked to see one more test run, with the window line 
>>> loss without any foam around it.
>>>
>>> The lower loss on the metal roof may be a measurement artifact - I'd 
>>> not be trusting 0.01 dB kinds of measurements.  The metal roof is 
>>> "mostly" farther away than the thickness of the foam, except where 
>>> the roof ribs stick up.
>>>
>>> 'NBC didn't give the length of his line, but if we work from the 19.8 
>>> MHz half wave, that's about 7 meters (25 feet?), so he's taking the 
>>> measurement and multiplying by 4 to get the "per 100 ft" - so 0.01 dB 
>>> on the graph is really 0.0025 dB in the measurement.  (the picture 
>>> over the chairs is consistent with 25 feet)
>>>
>>> I'd say that metal and free air are "the same within experiment 
>>> uncertainty"
>>>
>>>
>>> One cool thing here is that measurement tools like the VNA are common 
>>> enough now that we can actually *measure* the losses, rather than 
>>> speculating or using dialectic (aka arguing on forums, letters to the 
>>> editor, and email lists).
>>>
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>>
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